The present invention relates to the art of combustion methods and apparatus. The invention finds particular application in conjunction with furnaces and will be described with reference thereto. It is to be appreciated, however, that the invention is equally applicable to many combustion installations including boilers, kilns, and other heating apparatus.
Heretofore, industrial furnaces have included a combustion chamber in which a plurality of burners were located. Amplitude modulated control systems were utilized to control the temperature within the combustion chamber. Specifically, a controller would sense the temperature within the combustion chamber, compare the sensed temperature with a selected temperature, and control the amount of fuel and air supplied to the burners. In this manner, the burners combusted fuel at a varied rate to maintain or reach a selected temperature.
One of the problems with the amplitude modulated furnace systems is that they are relatively fuel inefficient. Physical attributes and limitations of the prior art burners caused them to obtain a peak combustion efficiency at a specific or small range of air-to-fuel ratios. When the burners combust fuel either more or less rapidly than the peak efficiency air/fuel ratio, they operate with relatively less fuel efficiency. Further, it is difficult to maintain a stoichiometrically balanced air/fuel ratio over a wide range of air and fuel supply rates. Another problem with the amplitude modulated burner systems has been that at reduced heats, they have relatively low conductive heat transfer characteristics. Particularly, the heated gases have less momentum at lower temperature settings, i.e., at lower combustion rates. Further, the burners are frequently unable to maintain stable flames over a wide range of heating rates.
In accordance with the present invention, a frequency modulated control system with two-stage burners for combustion apparatus is provided to overcome the above-referenced problems and others, yet reliably maintain an accurate temperature control with high fuel efficiency.